Among the numerous functions attributed to vitamin D, current evidence suggests that its primary function is to facilitate the processes that are essential for the maintenance of a healthy and mineralized skeleton. This hormone plays a key role in calcium and phosphate homeostasis and a deficiency would lead to resorption of bone, osteoporosis, and reduced bone mineralization. In this way, this vitamin has been linked to many bone diseases, including osteoporotic hip fractures. Moreover, vitamin D improves bone mineralization in patients with chronic renal failure, supporting the fact that vitamin D directly promotes bone mineralization. Other different effects of vitamin D have been defined as anticancer and also having immunomodulatory actions. However, the use of the active vitamin D could not be applied in bone repair because of its detrimental hypercalcaemic action and toxicity.
It is well established that ultraviolet irradiation is the major source of vitamin D synthesis in the skin. 7-Dehydrocholesterol (7-DHC) is a photolabile cholesterol precursor that is converted to precholecalciferol (previtamin D3) when exposed to ultraviolet B sunlight. The latter is transformed into cholecalciferol (D3) and transported to the liver where is hydroxylated at carbon 25 on the side chain by vitamin D3 25-hydroxylase (CYP27A1) to form the major circulating intermediary, 25-hydroxyvitamin D3 (25-D3). Renal hydroxylation by 25-hydroxyvitamin D3-1alpha-hydroxylase (CYP27B1) transforms the previous form into the biologically active steroid hormone, 1,25-dihydroxyvitamin D3 (1,25-D3) that is released to the circulation.
It is well known that human skin cells exposed to ultraviolet B (UVB) radiation (spectral range 290-315 nm) convert 7-DHC into previtamin D3 in vivo and in vitro. A UVB wavelength-dependent synthesis of vitamin D3 was found in these cells, showing a maximum 1,25-D3 ratio formation at 302 nm and no vitamin D3 production when UV wavelengths >315 nm were used (Lehman et al. UVB-induced conversion of 7-dehydrocholesterol to 1α,25-dihydroxyvitamin D3 in an in vitro human skin equivalent model. Journal of Investigative Dermatology 2001; 117:1179-85). However, there is no study supporting the same activity for osteoblastic cells with 7-DHC.
Osseointegration refers to a direct structural and functional connection between living bone cells and the surface of a load-bearing implant. Titanium (Ti) is the material most commonly used for bone implants as it has outstanding physical and biological properties, such as low density, high mechanical strength, and good corrosion resistance. Current dental implant research aims at the production of innovative surfaces able to promote a more favorable biological response to the implant material at the bone-implant interface and to accelerate osseointegration. Surface preparations, chemical composition or coatings are procedures that may affect the stability of the bone-to-metal interface.